Power Quality Benchmarking
Electric utilities throughout the world are embracing the concept of benchmarking service quality. Utilities realize that they must understand the levels of service quality provided throughout their distribution systems and determine if the levels provided are appropriate.
This is certainly becoming more prevalent as more utilities contract with specific customers to provide a specified quality of service over some period of time. The typical steps in the power quality benchmarking process are
- Select benchmarking metrics. The EPRI RBM project defined several performance indices for evaluating the electric service quality. A select group are described here in more detail.
- Collect power quality data. This involves the placement of power quality monitors on the system and characterization of the performance of the system. A variety of instruments and monitoring systems have been recently developed to assist with this labor-intensive process.
- Select the benchmark. This could be based on past performance, a standard adopted by similar utilities, or a standard established by a professional or standards organization such as the IEEE, IEC, ANSI, or NEMA.
- Determine target performance levels. These are targets that are appropriate and economically feasible. Target levels may be limited to specific customers or customer groups and may exceed the benchmark values.
The benchmarking process begins with selection of the metrics to be used for benchmarking and evaluating service quality. The metrics could simply be estimated from historical data such as average number of faults per mile of line and assuming the fault resulted in a certain number of sags and interruptions. However, electricity providers and consumers are increasingly interested in metrics that describe the actual performance for a given time period. The indices developed as part of the EPRI RBM project are calculated from data measured on the system by specialized instrumentation.
Electric utilities throughout the world are deploying power quality monitoring infrastructures that provide the data required for accurate benchmarking of the service quality provided to consumers. These are permanent monitoring systems due to the time needed to obtain accurate data and the importance of power quality to the end users where these systems are being installed. For most utilities and consumers, the most important power quality variation is the voltage sag due to short-circuit faults. Although these events are not necessarily the most frequent, they have a tremendous economic impact on end users. The process of benchmarking voltage sag levels generally requires 2 to 3 years of sampling. These data can then be quantified to relate voltage sag performance with standardized indices that are understandable by both utilities and customers.
Finally, after the appropriate data have been acquired, the service provider must determine what levels of quality are appropriate and economically feasible. Increasingly, utilities are making these decisions in conjunction with individual customers or regulatory agencies. The economic law of diminishing returns applies to increasing the quality of electricity as it applies to most quality assurance programs. Electric utilities note that nearly any level of service quality can be achieved through alternate feeders, standby generators, UPS systems, energy storage, etc. However, at some point the costs cannot be economically justified and must be balanced with the needs of end users and the value of service to them.
Most utilities have been benchmarking reliability for several decades. In the context of this book, reliability deals with sustained interruptions. IEEE Standard 1366-1998 was established to define the benchmarking metrics for this area of power quality. The metrics are defined in terms of system average or customer average indices regarding such things as the number of interruptions and the duration of interruption (SAIDI, SAIFI, etc.). However, the reliability indices do not capture the impact of loads tripping off-line for 70 percent voltage sags nor the loss of efficiency and premature equipment failure due to excessive harmonic distortion.
Interest in expanding the service quality benchmarking into areas other than traditional reliability increased markedly in the late 1980s. This was largely prompted by experiences with power electronic loads that produced significant harmonic currents and were much more sensitive to voltage sags than previous generations of electromechanical loads. In 1989, the EPRI initiated the EPRI Distribution Power Quality (DPQ) Project, RP 3098-1, to collect power quality data for distribution systems across the United States. Monitors were placed at nearly 300 locations on 100 distribution feeders, and data were collected for 27 months. The DPQ database contains over 30 gigabytes of power quality data and has served as the basis for standards efforts and many studies. The results were made available to EPRI member utilities in 1996.
Upon completion of the DPQ project in 1995, it became apparent that there was no uniform way of benchmarking the performance of specific service quality measurements against these data. In 1996, the EPRI completed the RBM project, which provided the power quality indices to allow service quality to be defined in a consistent manner from one utility to another. The indices were patterned after the traditional reliability indices with which utility engineers had already become comfortable. Indices were defined for
- Short-duration rms voltage variations. These are voltage sags, swells, and interruptions of less than 1 min.
- Harmonic distortion.
- Transient overvoltages. This category is largely capacitor-switching transients, but could also include lightning-induced transients.
- Steady-state voltage variations such as voltage regulation and phase balance.
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